Markers are used today for visualizing where a tumour is or has been located with for example x-ray. The marker is placed in a needle that is sterilized. The tip of the needle is advanced into the tumour and the marker is pushed out of the needle with a mandrin, i.e. a wire movable inside a cannula or pipe.
In connection with surgery, a so called clip may be attached to the tissue where the tumour has been. The markers may be left in the body and are often of some sort of inert material with high density in order to facilitate the imaging with the help of x-ray.
In connection with radio-therapy, the patient is often positioned with the aid of markers on the skin. This leads to great uncertainty regarding positioning due to movement of the skin in relation to inner organs. By producing x-ray images the skeleton may give guidance regarding the position of the therapy ray. Inner organs move also in relation to the skeleton and skeleton x-ray does not always give the correct guidance regarding the position of the tumour at the actual time when the x-ray image is taken. Soft tissue is not recorded with conventional x-ray. This may be done with the aid of computer tomography.
Ideally it is desirable to be able to position the tumour securely in relation to the interception point of the central beam from different directions of radiation, the point which is called the iso-centre. If this can be done, large safety margins do not have to be added, which margins may amount to several centimeters. The volumes of the margins often become large in relation to the volume of the tumour, a volume of normal tissue that should no be radiated. Markers on the skin may be relatively accurate in 75 percent of the radiation cases while precision radiation requires refined methods. Several methods are on the process, for example breath gating, which means that the location of the tumour is estimated during different breathing phases. This is an indirect method that is time consuming and ineffective regarding resources and precision. There are equipment today that enables computer tomography on the radiation treatment table for locating the tumour at each treatment occurrence. This is also partly time consuming and requires the presence of a physician. The only direct method for positioning of tumours with precision, that also is cost-effective, is markers.
Markers in the tumour or in its vicinity is a valuable aid for positioning. The drawback is that it requires a certain mass in order to retard high energy x-ray beams for sufficient contrast on films or portal images, which leads to that the marker has to be relatively large. It requires a relatively thick needle that cannot be entered easily in any part of the body. A usual dimension of a gold marker is 1.0×3.0 mm. Such large needles for positioning markers can not penetrate all parts of the human body without the risk of internal bleeding, infection and the need for anaesthesia.
Markers are however relatively new in use. The therapy beam with energies of 4-50 MV (megavolt) provides a weak contrast of skeleton parts, providing difficulties in evaluating skeleton and markers during treatment. Several of the manufacturers of accelerators used during treatment have developed conventional x-ray add-ons on the accelerators. With this technology new possibilities and a new market are created for x-ray dense markers. High density retards x-ray radiation very well. A marker of silver has enough density for kilovolt x-ray, providing a good contrast but 24 carat gold is soft and is very suitable in this aspect. If one wants a visible marker with a therapy beam of several megavolt, then it is the mass that is important and not the density itself. In this aspect gold is more suitable than silver.